In electrostatic drum printing on sheets, such as in facsimile machines, an electrostatic latent image on the sheets is developed into a visible image, usually by applying a liquid suspension of toner particles to the sheets. The developer typically must remain in contact with the sheets for about one second to obtain an adequate image contrast. Attempts have been made to increase the rate of printing by increasing the speed of rotation of the drum. However, this sometimes results in a loss of image contrast or density.
In U.S. Pat. No. 4,482,242 to Moraw et al., a latent charge image on a drum is developed by moving the drum through a pool of developer liquid in a developing shoe. The excess liquid is removed by spillage off the edges of the shoe, and by a fast moving roller contacting the drum.
U.S. Pat. No. 4,270,859 to Galbraith et al. provides a pre-wet shoe for depositing a liquid dispersant onto photoconductive paper before development, and a toner shoe adjacent to the paper for applying toner to a latent image on the paper. Toner is applied across the width of an entire sheet. The toner fluid flows in the shoe to the paper, then downhill along the paper to an outlet of the shoe. The toning shoe is raised by a lifting mechanism to a prescribed distance from the drum surface, and the shoe does not contact the drum.
In copending U.S. application Ser. No. 763,040 assigned to the assignee of the present invention, a drum printer features a print element or head with a linear array of closely spaced charging elements for writing an electrostatic latent image helical stripe pattern on a sheet mounted on a drum. When the sheet is laid out, the stripe pattern consists of adjacent columns of digital multi-bit words forming an image. Such print heads are faster than those which only print a single helical raster line at a time, yet without the expense involved with full width line printer heads.
It is desirable to devise a development electrode that allows the use of a print head with helical stripe scanning thereby obtaining a faster printing speed. An even faster print speed could be obtained if the time of fluid developer contact with the sheet could be reduced to less than a full second. However, it is imperative that vital image contrast not be lost. Also streaking and uneven developing must be avoided.
An object of the invention is to devise a developer applicator for a supported latent image, particularly a structure which allows rapid helical stripe pattern printing without streaking or uneven imaging at stripe edges.